The invention relates to a drilling motor and a system and method for drilling curved boreholes.
To drill curved boreholes, it is known to use a system as shown in FIG. 1 of the drawing. A downhole drilling motor 1 is connected at its lower end to a drill bit 2 and at its upper end by means of a bent sub 3 with bend angled to non-rotating drillpipe 4. Drilling fluid is pumped down the drillpipe 4 to drive the drilling motor 1 that rotates the drill bit 2. The drilling fluid passes through the drill bit 2 to cool and lubricate it and to carry the drill cuttings to the surface through the annular space between the drillpipe 4 and the borehole wall 5. The curvature of the borehole is obtained with the bent sub 3, that causes the drillpipe 4 not only to exert a longitudinal force on the drill bit 2, but also a sideways force. To obtain a curvature in the desired direction at the start of the curved borehole section, the bent sub 3 must be orientated in the desired direction.
Conventionally, the downhole drilling motor 1 is a positive displacement motor (PDM) based on the Moineau principle: A rotor with a single external helix rotates inside a stator containing an internal double elastomeric helix. This arrangement creates a series of cavities. When drilling fluid is pumped down the space between rotor and stator, these cavities progress downward, turning the rotor.
The problem of borehole spiralling in curved boreholes can be eliminated or minimized by using systems as shown in FIG. 2 of the drawing. A shorter downhole positive displacement drilling motor 1xe2x80x2 is directly attached to a long gauge bit 2xe2x80x2 (FIG. 2a) or is attached to a short gauge bit 2 by means of a long gauge sub 6, at its circumference equipped with one or more longitudinal recesses 7 for passage of the return drilling fluid back to the surface (FIG. 2b). These short downhole positive displacement drilling motors are preferably of the roller vane type with single jacket, with all the drilling fluid passing inside this jacket to the drill bit, as described in WO 99/20904 (PCT/NL98/00598), which give a similar performance as the much longer Moineau drilling motors.
As a result, a curved borehole can be drilled with the same radius of curvature with a much smaller bend angle in the bent sub 3. In that way, the problem of too much sideways force on the drill bit is eliminated and little or no spiralling occurs in the curved borehole section.
A problem with this system of curved drilling is that, when using the rather long downhole drilling motors of the Moineau type, the curved section is somewhat spiral-shaped, which slows down the drilling process. To solve this problem, attempts have been made to stabilize the drill bit by placing a long gauge sub between the drill bit and the drilling motor, or by using long gauge drill bit. Both methods failed, because it proved impossible to start or retain a borehole of the desired curvature under these circumstances.
The invention provides a system for downhole drilling and coring, said system comprising a roller vane motor for driving a drill bit, said motor having a single jacket with a lower end and an upper end and being embodied such that all drilling fluid passes inside said jacket to the drill bit.
The system further has a bent sub connected to the upper end of the motor. The jacket is provided with outwardly projecting curved members forming recesses therebetween for the passage of return drilling fluid, the curved members creating a circular surface having a diameter approximately equal to that of the drill bit.
The curved members as well as the recesses therebetween each extend in longitudinal direction over the full length of the jacket of the motor.
Preferably the curved members are extended to beneath the lower end of the jacket to near the drill bit.
Preferably the motor has wing deflector cams forming part of the jacket, a longitudinal recess being provided at the outer surface of the jacket opposite the wing deflector cam. Also the jacket has above and below the wing deflector cams an internal diameter approximately equal to that of the wing deflector cams.
The invention also relates to a method for drilling and coring curved and horizontal borehole sections, wherein use is made of a system according to one or more of the preceding claims and wherein the bent sub is connected to a non-rotating drill pipe.
The invention also relates to a roller vane motor for driving a drill bit, said motor having a single jacket with a lower end and an upper end and being embodied such that all drilling fluid passes inside said jacket to the drill bit, the jacket being provided with outwardly projecting curved members forming recesses therebetween for the passage of return drilling fluid, the curved members creating a circular surface having a diameter approximately equal to that of the drill bit.
The curved members as well as the recesses therebetween each extend in longitudinal direction of the jacket of the motor, and in that the motor has wing deflector cams forming part of the jacket, each longitudinal recess being provided at the outer surface of the jacket opposite a wing deflector cam.
The jacket above and below the wing deflector cams has an internal diameter approximately equal to that of the wing deflector cams.